Photoconductivity

Photoconductivity is an optical and electrical phenomenon in which the conductivity of a material increases as a result of absorbing electromagnetic radiation. When photons with sufficient energy interact with a solid—most commonly a semiconductor—they generate additional charge carriers, causing the material to conduct electricity more effectively. This effect underpins a wide range of photodetector technologies and has important applications in optoelectronics, imaging, sensing and data storage.

Fundamental Principles and Mechanism

In semiconductors and other photoconductive materials, conductivity depends on the number of free electrons and electron–hole pairs available for electrical conduction. When the material absorbs light in the visible, ultraviolet, infrared or other energetic regions of the spectrum, electrons can be excited across the band gap or from impurity levels into the conduction band. Light must therefore possess enough energy to raise electrons from bound states into free, mobile states.
A photoconductive device is typically placed in an electrical circuit with a bias voltage and a load resistor. Changes in conductivity alter the current flowing through the material, producing a measurable voltage change across the load resistor. This simple configuration makes photoconductive materials effective as variable resistors governed by light intensity.
Classic examples include silver halide compounds such as silver sulphide and silver bromide, historically used in photographic films such as Kodachrome, Fujifilm and Agfachrome. Organic photoconductors, such as poly-N-vinylcarbazole, have played a major role in xerography and early photocopying technologies. Inorganic compounds used in infrared detection—such as those incorporated into systems like the Sidewinder missile—illustrate the breadth of photoconductor applications. Molecular photoconductors may be organic, inorganic or, less commonly, coordination complexes.

Applications of Photoconductive Materials

When employed in electronic circuits, photoconductive materials function as resistors whose resistance depends on light intensity. In this context they are known as photoresistors or light-dependent resistors (LDRs). They constitute one family of photodetectors alongside charge-coupled devices, photodiodes and phototransistors.
Common applications include:

• camera light meters,
• automatic street lighting,
• alarm clocks and radios,
• infrared detectors,
• nanophotonic devices,
• low-dimensional photosensors.

These devices often leverage the simplicity and reliability of photoconductive components, especially where broad spectral response or low manufacturing cost is advantageous.

Sensitisation Techniques

Sensitisation is a key engineering procedure used to enhance the photoconductive response. Photoconductive gain is tied to the lifetime of photoexcited carriers: the longer the carriers persist before recombining, the greater the measurable current. Sensitisation involves deliberate doping of the material to saturate native recombination centres that exhibit short lifetimes and introduce new centres with longer lifetimes.
Correctly applied, these modifications can increase photoconductive gain by several orders of magnitude. Sensitisation techniques are central to the manufacture of commercial photoconductive devices. The authoritative reference on the subject is the work of Albert Rose, whose 1963 text on photoconductivity remains influential.

Negative Photoconductivity

Although illumination usually increases conductivity, some materials show the opposite behaviour, known as negative photoconductivity. In these systems, exposure to light reduces conductivity rather than enhancing it.
A notable example is hydrogenated amorphous silicon, which demonstrates a metastable decrease in photoconductivity, a phenomenon associated with the Staebler–Wronski effect. Other materials reported to exhibit negative photoconductivity include:

• zinc oxide,
• molybdenum disulphide,
• graphene,
• indium arsenide nanowires,
• carbon nanotubes with surface decorations,
• metal nanoparticles.

Zinc oxide nanowires display a particularly striking behaviour: under ultraviolet illumination and an applied alternating voltage, they transition from positive to negative photoconductivity as a function of frequency. They also show a frequency-induced metal–insulator transition at room temperature. These effects arise from the competition between bulk and surface conduction, a characteristic seen in semiconductor nanostructures with large surface-to-volume ratios.

Magnetic Photoconductivity

In 2016 researchers demonstrated that some photoconductive materials can exhibit magnetic ordering. A prominent example is the compound CH₃NH₃MnPbI₃, which shows light-induced melting of magnetic order. This dual optical–magnetic behaviour suggests possible applications in magneto-optical devices and next-generation data storage technologies, where light might be used to manipulate magnetic states.

Photoconductivity Spectroscopy

Photoconductivity spectroscopy, also known as photocurrent spectroscopy, is an important characterisation technique used to investigate optoelectronic properties of semiconductors. By measuring variations in photocurrent as a function of incident photon energy, researchers can determine band structure features, defect states and carrier dynamics. This method is widely applied in the study of emerging materials, including low-dimensional semiconductors and novel photovoltaic compounds.